A continuing program in laboratory animal nutrition involves studies with the species that are most frequently used as animal models in biomedical research. The objective of the program is to improve the nutritional status of NIH production and research colonies of rats, mice, guinea pigs, rabbits, dogs, cats and various species of non-human primates. A series of factorially-designed feeding trials are used to ascertain the nutrient requirements of the species of interest and to develop diets with more nearly optimal nutrient concentrations for their growth, reproduction, maintenance or general health status. This program has resulted in the development of a series of open formula natural ingredient diets that are not only used in animal colonies at NIH, but also in research animal colonies throughout its biomedical research community.